1. Field of the Invention
This invention relates to a starting device for lighting a discharge tube, particularly a fluorescent discharge tube. More particularly, this invention relates to a starting device which is capable of not only efficiently lighting a discharge tube of varying kind but also allowing adjustment of light over a wide range which, when used for lighting a plurality of discharge tubes, enables the individual discharge tubes to be sequentially started and consequently aids in attaining the purpose of decreasing weight, compacting structure, and lowering cost.
2. Description of the Prior Art
Generally, the fluorescent discharge tube in the course of actual service exhibits a property of refusing to start discharge unless a voltage several times the voltage used during a continued discharge is applied at the time of starting discharge and a negative property of retaining the voltage between the opposite terminals closely to a fixed magnitude during the continuous discharge in spite of an increase in feed current. The fluorescent discharge tube, therefore, requires a starting device which is provided with a function of applying between the opposite terminals a voltage exceeding the voltage of the continued discharge at the time of starting discharge and a function of regulating the current flowing to the fluorescent discharge tube and, at the same time, stabilizing the incoming current so as to overcome a possible variation in the voltage of the power source after the start of discharge.
Most of the starting devices for discharge tubes currently in popular use make direct use of commercial frequency power source. In terms of operating principle, those of small capacity start a fluorescent discharge tube by means of a glow lamp and a choke coil (stabilizer) and those of medium or large capacity instantaneously start a fluorescent discharge tube by virtue of the actions of a heater circuit and a high-voltage circuit incorporated in a special winding wrapped round a stabilizer. Very recently, starting devices which incorporate an electronic circuit therein and start a fluorescent discharge tube by virtue of the high-frequency voltage issued by the electronic circuit as disclosed in the specification of Japanese Utility Model Application Disclosure SHO 63(1988)-18,797 have been finding general acceptance.
In the conventional starting devices described above, however, those of small capacity requires use of a choke coil of relatively large capacity because they are adapted to light a fluorescent discharge tube with the aid of a glow lamp. Thus, they attain a desired reduction in size and weight only with great difficulty. By the same token, those of medium to large capacity allow the reduction only with extreme difficulty. Any attempt at conferring such a highly advanced function as regulation of light upon any of the conventional starting devices is substantially impracticable on account of the characteristic construction of the device. It has been difficult to materialize the regulation of light and consequently efficient start of the fluorescent discharge tube.
Further in the case of the starting device which incorporates an electronic circuit therein as described above, the regulation of light mentioned above is attained to some extent. When one and the same device is relied on to effect the regulation of light stably and, at the same time, allow efficient start of the discharge tube, the range over which the stable regulation of light is obtained is not very wide.
Further, the conventional starting devices are adapted to serve exclusively for fluorescent discharge tubes of their own allocations. When they are manufactured at a factory, for example, they must be produced in numerous types including those for exclusive use with 40-W linear type fluorescent discharge tubes and those for exclusive use with 20-W circular type fluorescent discharge tubes.
When the conventional starting devices are relied on each to light a plurality of discharge tubes, it becomes necessary for the plurality of discharge tubes to be severally provided with a starting device or to be furnished collectively with a power source and a rectifying circuit both large in size and capacity, with the result that the entire system suffers from an addition to size and an increase in power consumption. Further, the rush current of a large magnitude which occurs at the time of starting exerts a load on inner circuit elements. The repetition of this exertion of the load possibly entails disruption of such circuit elements and eventual failure of the device. This attempt, therefore, proves to be highly disadvantageous in terms of weight, capacity, cost, power consumption, etc.